It completely depends on the situation, and the guys personality. If he is flirty and using you, then no. If you are still into him, and he tells you he made a mistake, and you belive he is being genuine and honest, then possibly. My last relationship was really hard to get over. However, now I am.
My ex showed up uninvited at my sisters grad party a week and a half ago, asking if I still had feelings for him. I told him no and we have to move on. If this was three months ago, I probably would have said yes. However in the last three months I met a great guy and I'm in a relationship sooo, I have moved on. My ex is a really great guy, I don't harbor any anger towards him, he has his faults and so do I, but, break-ups tend to happen for a reason.